I just did that level on vet, it was a killer, took me a good 40mins to get past that 1 part. I found in the 30second prelude drop claymores all over, next to vehicles to add extra boom for your buck.
I then layed down near mcmillan got out my sniper, then took out alot of the 1st group, dogs seemed to kill the rest

.
next wave I sniped again, had 1 or 2 left when the helicopters arrived. I hid behind the hut to the left of the ferris weel and used ak to take out everyone from the 2nd and 3rd helicopter.
I then sprinted to the bumper cart area and hid behind that to then kill the remaining people with AK.
The copter landed I ran picked up mcmillan and left.
and ye its a git when you run outta time for things like that

although I do have to say that the sniping mission was defo my fav mission so far